There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communication, satellites, portable computers and electric vehicles to name but a few. There have also been concerted efforts to develop high energy, cost effective batteries having improved performance characteristics, particularly as compared to storage systems known in the art.
Rechargeable, or secondary cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being regenerated (i.e. recharged) many times by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently, much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities of electrochemical cells.
Heretofore, electrolytes have been either liquid electrolytes as are found in conventional wet cell batteries, or solid films as are available in newer, more advanced battery systems. Each of these systems have inherent limitations, and related deficiencies which make them unsuitable for various applications.
Liquid electrolytes, while demonstrating acceptable ionic conductivity, tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells still do leak potentially dangerous liquid electrolytes from time to time. This is particularly true of current lithium ion cells. Moreover, any leakage from the cell lessens the amount of electrolyte available in the cell, thus reducing the effectiveness of the cell. Cells using liquid electrolytes are also not available for all sizes and shapes of batteries.
Conversely, solid electrolytes are free from problems of leakage. However, they have vastly inferior properties as compared to liquid electrolytes. For example, conventional solid electrolytes have ionic conductivities in the range of 10.sup.-5 S/cm, whereas acceptable ionic conductivity is &gt;10.sup.-3 S/cm. Good ionic conductivity is necessary to ensure a battery system capable of delivering usable amounts of power for a given application. Good conductivity is necessary for the high rate operation demanded by, for example, cellular telephones and satellites. Accordingly, solid electrolytes are not adequate for many high performance battery systems.
Examples of solid polymer electrolytes, include dry solid polymer systems in which a polymer, such as polyurethane, is mixed with an electrolyte salt in dry or powdered form. These types of systems are disclosed in, for example, Ionic Conductivity of Polyether-Polyurethane Networks Containing Alkali Metal Salts. An Analysis of the Concentration Effect, Macromolecules, Vol. 17, No. 1, 1984, pgs. 63-66, to Killis, et al; and Poly(dimethylsiloxane)--Poly(ethylene oxide) Based Polyurethane Networks Used As Electrolytes in Lithium Electrochemical Solid State Batteries, Solid State Ionics, 15 (1985) 233-240, to Bouridah, et al.. Unfortunately, these dry systems, like the solid electrolytes discussed above, are characterized by relatively poor ionic conductivity.
One solution which has been proposed relates to the use of so-called gel electrolytes for electrochemical systems. Gels, or plasticized polymeric systems are wet systems, not dry, as described above. Heretofore most gel electrolyte systems have been based on homopolymers, i.e, single polymer systems. Homopolymer-based gel electrolytes have not been successful as they tend to dissolve in higher concentrations of the electrolyte solvent, thus losing mechanical integrity.
Accordingly, there exists a need for a new electrolyte system which combines the mechanical stability and freedom from leakage offered by solid electrolytes with the high ionic conductivities of liquid electrolytes.